The present invention relates generally to well drilling operations and, more particularly, to well drilling operations using magnetic ranging while drilling to avoid collisions with existing cased wells.
With conventional drilling practices, the uncertainties in a well's position increase as the depth of the well increases. These uncertainties are usually represented as ellipsoids that are centered on the location of the well as determined by Measurement While Drilling (MWD) or wireline survey data. An ellipsoid corresponds to a certain probability density corresponding to whether the well bore is actually located within the ellipsoid. The uncertainties in the well position arise from the limited accuracy of the well bore direction, inclination, and depth measurements which may be obtained from MWD and/or wireline surveys, as documented extensively. For example, MWD inclination measurements are typically accurate to no better than 0.1°, while MWD directional measurements are typically accurate to no better than 1°. Moreover, MWD survey points may be acquired only once every 90 feet in practice. Thus, under-sampling may significantly increase the actual errors in the well position.
An additional source of survey error arises because the directional measurement is based on the magnetic field, which requires correction for variations in the Earth's magnetic field, and which can also be strongly perturbed by nearby casing. If the casings are very close to the well path, then the MWD directional measurement may not even be useful. Under such conditions, a gyro may be used to provide the directional information. The gyro may be run with the MWD tool, or it may be run on wireline with periodic descents inside the drill pipe to the bottom hole assembly (BHA). Finally, an accurate MWD depth measurement is difficult to achieve, with depth errors of 1/1000 common.
Further complications may arise in older fields with existing wells. In older fields, the survey information on existing wells may be very low quality, survey data may have been lost, or the wells may have been drilled without running a MWD or wireline survey.
Wells associated with a typical offshore platform are drilled vertically for a considerable depth before they are deviated to reach distant portions of the reservoir. These vertical sections typically range from several hundred feet to a few thousand feet before they reach the kick-off point (KOP) where directional drilling begins. Because offshore production platforms are very expensive and have as many wells as possible given the limited surface area of the platform, well heads are packed as closely as possible. The distances between well heads, and therefore the number of wells, are limited primarily by the uncertainty in well positions and the risk of accidentally drilling into a cased well. Since an existing cased well and the drill bit could be located anywhere inside the respective ellipsoids of uncertainty, well heads are spaced a distance apart so that any two ellipsoids cannot overlap.
Existing platforms may have filled many or all of the available slots (i.e., locations for well heads) based on factors derived from MWD direction and inclination technology. In order to tap additional oil or gas resources, new wells may be drilled. Unless there is a reliable method to avoid drilling into an existing well, another platform may have to be built. However, if one could thread new wells among the existing wells without risk of collision, then a new platform may not be needed.